It is believed that search engines may consider linking among sites belonging to same Class C block as triangular linking scheme and thus do not award credit to such links. From search engine optimisation perspective these links won’t be penalized though by the search engines.
First of all what are Class C Block of IPs? How would one know if the sites belong to same Class C block? Many get confused with Class C IP block as the 3rd octet in IP address; i.e. CC in AA.BB.CC.xx. This may not be technically correct.
The IPv4 addressing with respect to classful networks implies that the Class C addresses will have the first octet in 192 – 223 range. Here AA.BB.CC signifies the network ID and xx signifies the actual host ID. Thus a single Class C block would have 2,097,151 network addresses and each network address would have 254 host addresses. Hence AA.BB.CC put together signify a Class C Block and not just the third octet CC.
It is really difficult to acquire multiple IPs in different Class C Blocks, especially if one is hosting all the sites on the same hosting server. Although it is possible with large hosts that one may explicitly request the sites to have IPs on different Class C blocks. But by default, this is not taken care of.
If one back link comes from 195.12.1.1 and another comes from 195.12.1.2, they would be considered one and the same by the search engines. The value of these links may not be of much use and this is one the ways for the search engines to find related sites.
